You have not let anyone down least of all yourself or D. Remember it is not you that is doing this. Speaking personally this IS the hardest thing I have ever had to do in my life.
But you can do your best going forwards to change the outcome, and the worst that can happen is you will have an even better R with D and you will be a much better man. Learn from it, don't beat yourself up and move forwards - easier said than done I know but it is the only way.
You have 3 choices:
1. work on yourself, focus on youself, enjoy the time with D, GAL and do your 180's for you 2. Sit around and dwell on your sitch, focus on your W, make yourself ill, sink into depression 3. Give up, file and move on.
Only you can do this and only you can make that choice - and it is a choice. You basically sink or swim.
Write your bucket list of things you have always wanted to do and make some plans. 25yrsmlc has some great ideas about what she did in her sitch - it made me feel exhausted just looking at it, BUT it also made me realise I have no excuses to not GAL.
Keep exercising, eat properly and get plenty rest.
Your daughter will suffer more if she sees your pain. Plan a trip with her, spend quality time with her, show her that despite the sitch you are fine and that you and D can still have a fantastic time together. Remain upbeat around her as well as W. I know it [censored] and the hardest thing is the guilt about what it might be doing to your kids. But you show them a better more independent you which is all you can do.
Learn to accept your emotions and allow yourself to grieve (in private or with friends but not in front of W or D).
Use the boards to vent, update, get support and advice. There are some great people on here all who have been or are going through similar. They are proof it can be done and none of us could do this without the help on here.
Her moving out will also give chance for your W to notice your changes.
One other thing is to accept that W is not the W you knew. That woman has gone along with the M. She won't snap out of this and wake up. Its up to you to do the work and convince the new W that the new you and the potential for a new M are worth giving another shot.
I know this is tough and I still count myself as being fairly new to all this but have come along way since discovering DB.